The present invention relates generally to circuits that power load devices, and in particular to circuits that supply load devices with AC voltages derived from DC sources.
There are two types of electrical power. Direct current (DC) electrical power is characterized by its constant voltage and current. This is the type of power delivered, for example, by electrical storage cells, chemical batteries, and photovoltaic devices. Although typically used to power electrical devices, resistive losses proportionate to the square of its amperage render it undesirable for long-range power transmission.
Alternating current (AC) electrical power is typically characterized by time-varying voltage and current values whose time-average values are typically zero. Typically, its resistive losses are much smaller than those incurred through the transmission of DC power and therefore it is the power of choice for long-range power transmission. However, its varying voltage and current renders it unsuitable to power devices designed around logic levels that correspond to constant voltage levels. Therefore, most digital logic circuits are designed and operate on DC power.
However, certain circuit components, like electroluminescent panels and electrical motors, require AC power to operate due to design or device characteristics. Therefore, circuit designers are often faced with the problem of converting a supplied DC voltage to an AC voltage to power these devices. Additionally, certain equipment can have multiple loads which require separate control. For example, some cellular phones and personal digital assistants include multiple electroluminescent components and/or piezoelectric transducers which require separate power control. It is desirable to use a single DC power supply with minimal circuit components to individually control power to the loads of such devices.
The present invention relates to a circuit and method for powering AC devices using DC voltage sources. The present invention provides an improved circuit that requires fewer discrete components to power multiple devices. The circuit enables a user to selectively power a subset of the AC devices electrically connected to the present invention. The present invention also enables a user to switch the direction of the current flow through multiple load devices while reducing the number of components.
In one aspect, the invention relates to a circuit for driving multiple load elements. The circuit includes a first reference voltage terminal and a second reference voltage terminal. The circuit also includes a first, second, and third switch, each having an output terminal. The circuit also includes a first control switch in electrical communication with the first switch. The first control switch provides either a first control signal or a second control signal to a control terminal of the first switch. The circuit further includes a second control switch in electrical communication with the third switch. The second control switch provides either the first control signal or the second control signal to a control terminal of the third switch. The output terminal of the first switch is coupled to the first reference voltage terminal when the first control signal is in a first state. The output terminal of the first switch is coupled to the second reference voltage terminal when the first control signal is in a second state. Similarly, the output terminal of the third switch is coupled to the first reference voltage terminal when the second control signal is in a first state. The output terminal of the third switch is coupled to the second reference voltage terminal when the second control signal is in a second state.
In one embodiment, the first control switch has a selection terminal. A first selection signal applied to the selection terminal of the first control switch determines whether the first control switch provides the first control signal or the second control signal to the control terminal of the first switch. In a further embodiment, the second control switch also has a selection terminal. A second selection signal applied to the selection terminal of the second control switch determines whether the second control switch provides the first control signal or the second control signal to the control terminal of the third switch.
In another aspect, the invention relates to a method for powering multiple load elements. The method includes the step of providing a first load and a second load, each having a first load terminal and a second load terminal. The second load terminal of the first load is electrically coupled to the first load terminal of the second load. The method also includes the steps of selecting a power state or an off state for each load device and applying a first reference voltage to the second load terminal of the first load. The method includes the additional step of applying a second reference voltage to the first load terminal of the first load if the power state is selected for the first load. If the power state is selected for the second load, the second reference voltage is applied to the second load terminal of the second load. If the off state is selected for the first load, the first reference voltage is applied to the first load terminal of the first load. Similarly, if the off state is selected for the second load, the first reference voltage is applied to the second load terminal of the second load.